1. Field of the Invention
This invention is concerned with a process for catalytically dewaxing a hydrocarbon oil. In particular, it is concerned with dewaxing a hydrocarbon oil feedstock by contacting the feedstock with a catalyst comprising an intermediate pore size silicoaluminophosphate molecular sieve and at least one Group VIII metal.
2. Description of the Prior Art
Processes for dewaxing petroleum distillates are well known. Dewaxing is required when highly paraffinic oils are to be used in products which need to remain mobile at low temperatures, e.g., lubricating oils, heating oils, jet fuels. The higher molecular weight straight chain normal and slightly branched paraffins which are present in oils of this kind, are waxes which are the cause of high pour points and high cloud points in the oils. If adequately low pour points are to be obtained, these waxes must be wholly or partly removed. In the past, various solvent removal techniques were used, e.g., propane dewaxing, MEK dewaxing, but these techniques are costly and time consuming. Catalytic dewaxing processes are more economical and achieve this end by selectively cracking the longer chain n-paraffins, to produce lower molecular weight products, some of which may be removed by distillation.
Because of their selectivity, prior art dewaxing catalysts generally comprise an aluminosilicate zeolite having a pore size which admits the straight chain n-paraffins either alone or with only slightly branched chain paraffins, but which excludes more highly branched materials, cycloaliphatics and aromatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes and their use is described in U.S. Pat. Nos. 3,700,585; 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282 and 4,247,388.
Since dewaxing processes of this kind function by means of cracking reactions, a number of useful products become degraded to lower molecular weight materials. For example, waxy paraffins may be cracked down to butane, propane, ethane and methane and so may the lighter n-paraffins which do not, in any event, contribute to the waxy nature of the oil. Because these lighter products are generally of lower value than the higher molecular weight materials, it would obviously be desirable to limit the degree of cracking which takes place during a catalytic dewaxing process.
A dewaxing catalyst has now been found which has superior selectivity with respect to the nature of the products obtained in a dewaxing process. By use of certain silicoaluminophosphate molecular sieve catalysts in the dewaxing process, hydrocarbon oil feedstocks may be effectively dewaxed wherein the products obtained are of higher molecular weight than those obtained using the prior art aluminosilicate zeolites. Also, and especially with respect to lubricating oil feedstocks, the products obtained from the dewaxing process of this invention have better viscosities and viscosity indexes at a given pour point as compared to the above-described prior art process using aluminosilicate zeolites.